Acoustic method and system



llg- 21, 1956 J. J. BARucH 2,759,555

ACOUSTIC METHOD AND SYSTEM Filed July 2l, 1952 TEST CHAMBER FIG. 3

2o 3o Iomeovoaosmoo cYcLEs PER SECOND FIG. 2

FIG.

SIBHIOBG Nl NOILVDNBLLV INVENTOR. JORDAN J. BARUCH ATTORNEYS UnitedStates Patent ACOUSTIC METHOD AND SYSTEM Jordan J. Baruch, Cambridge,Mass., assignor, by mesne assignments, to Bolt, Beranek and Newman Inc.,Cambridge, Mass., a corporation of Massachusetts Application July 21,1952, Serial No. 300,113

24 Claims. (Cl. 181-50) The present invention relates to acousticmethods and systems, and, more particularly, to methods of and systemsfor reducing the intensity of sound energy accompanying the flow of afluid medium within a confined space.

It is frequently desired to eliminate or markedly reduce the intensityof sound energy accompanying the flow of a fluid medium, such as air,through a confined space. In test chambers, for example, for testing theperformance of aircraft engines and propellers, air passing through thechambers carries with it audible noise sounds produced by the operationof the engine and propeller of sufficient intensity to bring discomfortto the human ear. Several devices have been proposed for reducing thelevel of intensity of these disturbing audio-frequency components of thesound energy emitted from test chambers, but these are all attendantwith serious disadvantages.

Among these devices has been a terminal portion for the test chamberdivided into a plurality of ducts lined with sound-absorbing materialIand resonant at certain disturbing audio sound-frequencies in order toattenuate the said frequencies passing from the test chamber throughterminal-portion ducts. Such ducts are usually employed for attenuatingthe very low frequency audible sounds. In order to attenuate thesomewhat higher audio frequencies, the test chamber is provided withfurther portions containing baes. For use with the above-mentionedaircraft engine and propeller test chambers, however, the duct andbaffle portions must have a very considerable overall length, of fromabout thirty to seventy feet, in order to effect the desired result.This technique, in addition, does not lend itself to the scaling orproportioning of the dimensions of the sound-absorbing system fordifferent frequencies, as for the purpose of attenuating somewhatdifferent frequency bands. Such a structure, furthermore, is subject tohigh aerodynamic losses because of the twisting action upon the airflowing through the ducts and then through the bafiies. It is quitedifficult, moreover, to install such a system, and the masonry andsteel-supporting structure necessary to effect the same is quiteextensive.

Another technique proposed for solving this problem has been to utilizea labyrinth in which wall sections project from opposite sides of theterminal portions of the test chamber in order to provide a crooked pathfor the travel of both air and sound. These wall sections are providedwith sound-absorptive layers in order to attenuate the higher audiofrequency components of the sound energy which tend to travel instraight lines. Again, however, considerable expense and size isrequired to enclose a labyrinth at the ends of a standard test cell. Theaerodynamic losses attendant upon the forcing of the air through thiscrooked course, moreover, are high, and the turbulence introduced by theair passing through the labyrinth very often imposes severe rnechanicalstrains upon the propeller or other device being tested in the testchamber.

An object of the present invention is to provide a new and improvedmethod of and system for reducing the intensity of sound energy of aband of frequencies accompanying the fiow of a fluid medium through aconfined space that shall not be subject to the above-describeddisadvantages.

A further object of the invention is to provide an attenuator for thesaid band of frequencies which may occupy a relatively small space andnecessitate a minimum of installation work and expense in adapting thesame to standard test chambers.

Still another object of the present invention is to providestream-lining for the flow of the air or other fluid medium whileeffecting the above-described attenuat1on.

Other and further objects will be described hereinafter and will be moreparticularly pointed out in the appended claims.

In summary, the invention relates to a method of and system for causinga fluid medium containing sound energy of a band of frequencies totravel along a zig-zag path, absorbing the high sound frequencies of thesaid band of frequencies as the medium travels along the said zig-zagpath, and fixing the crests of the zig-zag path substantially inaccordance with the half wave-length of the intermediate soundfrequencies of the said band of frequencies in order to resonate thesaid intermediate frequencies, thereby to attenuate the same. Preferredconstructional details are treated hereinafter.

The invention will now be described in connection with the accompanyingdrawing,

Fig. 1 of which is a plan View of a test chamber embodying the presentinvention in preferred form, the roof of the test chamber being removed,

Fig. 2 is a graph illustrating experimentally obtained characteristicsof the chamber of Fig. l, and

Fig. 3 is a perspective view, partly broken away in order to showdetails, of a section of the preferred soundabsorbing device utilized inthe system of Fig. l.

Referring to Fig. 1, a test chamber 1 is shown centrally provided withan engine 3 for driving a propeller 5, for test purposes, mounted uponthe floor of the chamber in the customary manner. A door 7 in a sidewall of the test chamber 1 may be provided to permit access to the testchamber. At each end of the test chamber is disposed a terminal section9 provided with a plurality of longitudinally extending planarsound-absorbing devices 11. These sound-absorbing devices may be formedof fibre glass, rock wool, packed felt, or any other desiredsound-absorbing material that can be fabricated in sturdy sections. Thedevices 11 extend from the floor of the test chamber to the roof. Atperiodically spaced intervals, the otherwise uniform planar devices 11are provided with baffles 13 of the same acoustically absorbentmaterial. The baffles 13 thus form the largest transverse dimension ofthe sound-absorbing devices, and they are positioned so that the baffles13 of successive sound-absorbing devices are staggered across the testchamber. Thus, for example, the baies 13 of the second sound-absorbingdevice 11 in from either wall, as shown in Fig. l, are disposed betweenthe baffles 13 of the first and third sound-absorbing devices 11. While,in Fig. l, each sound-absorbing device 11 is shown provided with but twobaffles 13, this is merely for purposes of illustration, it beingunderstood that, in actual practice, two or more such baffles may beemployed depending upon the degree of attenuation and othercharacteristics desired. Air passing from the left of Fig. 1, throughthe left-hand terminal section 9, the central portion of the testchamber 1 and the right-hand terminal section 9, in the direction of thearrows, is thus forced,

as a result of this construction, to travel in zig-zag paths between theadjacent sound-absorbing devices llt-13.

Since the higher audio sounds generated by the engine 3 and/or propellerS tend to travel in straight lines, they become absorbed as the aircarrying the sound is forced to travel the said zig-zag paths betweenadjacent sound-absorbing devices 11-13. In Fig. 2, the dash-dot curve 2represents the variation of attenuation, produced by the effect of thesezig-zag paths, with frequency of the sound energy accompanying the iiowof air, attenuation being plotted as the ordinate in units of decibels,`and the frequency, as the abscissa, in units of cycles per second.While this zig-zag path satisfactorily attenuatcs the higheraudio-frequency components, say in the neighborhood of 600 to 1000cycles per second, the attenuation in the intermediate-frequency regionof, for example, 105i to 500 cycles per second, is not satisfactory toproduce the desired degree of elimination of sound in this frequencyrange. At 100 cycles, for example, only about ll decibels of attenuationare provided; at 200 cycles, about 20 decibels; while at 500 cycles,about 50 decibels of attenuation are produced. In accordance with thepresent invention, the intermediate sound frequencies of from about 100to about 500 cycles, are further attenuated by proper spacing of thelongitudinal separation L of the baffles i3 of each device lll13. Byspacing the successive crests, namely the successive direction-changingpoints, of the big-zag paths, to correspond substantially to the halfwave-length of these intermediate sound frequencies, such furtherattenuation is achieved. ln actual practice, an average or meanwavelength of the band of intermediate frequencies may be employed. Theuse of such periodic or tuned half-wavelength spacingot' thesound-absorbent baflles 13 of adjacent sound-absorbing devices ll ofitself produces the plurality of peaked attenuation characteristicsshown in the three dotted curves It, 5 and 3 of Fig. 2. rThis periodicstructure, in effect, causes resonance, actually multiple resonances 4,6 and 8, of the intermediate sound frequencies in the space betweensuccessive bales, thereby effecting further attenuation by theacoustically absorbent material of the devices l1-13. The operation ofthe system may be more easily understood by considering that thetransversely extending baffles i3 provide constricticns to the soundenergy traveling between the longitudinally extending members ll sincethe transverse separation between adjacent devices il becomes reduced atthe baies 13. Stich constrictions present an increase in impedance tothe dow of the sound energy. When such impedance increases are made tooccur periodically at successive intervals corresponding substantiallyto the half-wavelength of particular-requencied sound energy, ormultiples thereof, there is produced at such successive intervals a veryhigh overall impedance to the flow of that sound energy, thusappreciably further attenuating such sound energy, as illustrated at 4,6 and 8. In the tests plotted in Fig. 2, these multiple resonancecharacteristics are produced by this structure in the neighborhood ofabout 30 cycles, about 252l cycles, and about 500 cycles.

The resultant effect of the before-mentioned zigzagpath attenuation 2,and the periodic spacing attenuation il, 6, S, is plotted as thesolid-line curve l0 of Fig. 2. lt will be observed that the intermediatefrequencies from about 1GO cycles up to about 500 cycles are appreciablyattenuated with from about 28 to about 58 decibels of attenuation, andthat the high frequencies of the band above about 600 cycles areattenuated with about 58 decibels of attenuation. High attenuation isnot so iniperative for the very low frequencies, below about 100 cycles,inasmuch as they are not so disturbing to the human ear.

Though the before-described resultant attenuation characteristic i0 issatisfactory insofar as the acoustic results are concerned, the baies 13present an obstruction to the flow of air. As a result of the structureof the apparatus employed in accordance with the present invention,however, this obstruction is easily overcome by securing an acousticallytransparent septum 15 between the baffles 13 and, for example, themid-regions i7 of the sound-absorbing devices 11 disposed betweenadjacent baffles i3. This acoustically transparent septum may, as anillustration, assume the form of Wire screening. A streamlined zig-zagpath, preferably of substantially uniform width S, is thus providedpermitting air tiow without obstruction, but allowing the sound to passt.roiigli the septum l5 to the absorbing material of the devices ll-3 inorder to produce the before-described resultant attenuation effect 10,Fig. 2.

As an illustration of a typical installation in a standard aircraftengine test chamber for producing results similar to those shown in Fig.2, the transverse spacing S defining the width of the Zig-zag pathsbetween the longitudinally extending sound-absorbing devices 11--13 muyhave a value of from about 2 to about 6 feet, and the longitudinalseparation L of the successive battles 13 of each device 11-13 may havea value from about 4 to about l2 feet.

ln accordance with a further feature of the presentinvention, if it isdesired to shift the resultant attenuation curve l0 to the left or tothe right in order to cover lower or higher frequency bands for anyparticular application, it is merely necessary correspondingly to scaleor proportionately reduce or increase the before-mentioned dimensions,thereby automatically to produce the desired attenuation characteristiclt) over the desired frequency band. This is extremely convenientinasmuch as a single design of an installation, such as that illustratedin Fig. l, will per-mit the adaptation of the design to other frequencybands by this simple proportionate scaling process alone.

While the invention has been described in connection with a test chamberfor aircraft engines and propellers, it is to be understood that thesetechniques are equally applicable to other systems in which a fluidmedium containing undesired sound energy is passed through a confinedspace, such as, for example, in air-conditioning plants or in muiersystems, and the like. It is also to be understood that it is notessential that the devices 11- 13 have the strictly planar or strictlyrectangular contiguration illustrated in the drawing, but that othershapes may similarly be employed to provide the desired zig-zag pathsand the desired periodic resonant spacing of the sticcessively disposedballes.

Other and further modifications will occur to those skilled in the artand all such are considered to fall within the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:

l. in an acoustic system in which a fluid medium is adapted to flowlongitudinally through a confined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the flowof the medium comprising a plurality of longitudinally extendingsoundabsorbing devices spaced transversely within the conlfined spaceand shaped to provide for the sound energ a zig-zag path or pathsthere-between of varying transverse dimensions, whereby the high soundfrequencies of the said band of frequencies become attenuated during'the travel of the medium along the zig-zag path or paths,

successive crests of the zig-zag path or paths being spacedlongitudinally a distance corresponding substantially to thehalf-wavelength of intermediate sound frequencies of the said band offrequencies in order to attenuate the said intermediate frequencies.

2. In an acoustic system in which a fluid medium is adapted to owlongitudinally through a coni-ined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the Howof the medium comprising a plurality of longitudinally extendingsoundabsorbing devices each of periodically varying transverse dimensionspaced transversely within the confined space with the largesttransverse dimension of each device disposed substantially opposite tothe smallest dimension of the device or devices disposed adjacentthereto, whereby the high sound frequencies of the said band offrequencies become attenuated during the travel of the medium along theresulting zig-zag path or paths, the largest transverse dimensions ofeach device being separated from one another a distance correspondingsubstantially to the half-Wavelength of intermediate sound frequenciesof the said band of frequencies in order to attenuate the saidintermediate frequencies.

3. In an acoustic system in which a fiuid medium is adapted to flowlongitudinally through a confined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the iiowof the medium comprising a plurality of longitudinally extendingsoundabsorbing devices each of varying transverse dimension spacedtransversely within the confined space with the largest transversedimension of each device longitudinally displaced from the largesttransverse dimension of the device or devices disposed adjacent thereto,whereby the high sound frequencies of the said band of frequenciesbecome attenuated during the travel of the medium along the resultingZig-zag path or paths, the largest transverse dimensions of each devicebeing separated from one another a distance corresponding substantiallyto the halfwavelength of intermediate sound frequencies of the said bandof frequencies in order to attenuate the said intermediate frequencies.

4. In an acoustic system in which a fluid medium is adapted to flowlongitudinally through a confined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the fiowof the medium comprising a plurality of longitudinally extendingsound-absorbing devices each of varying transverse dimension spacedtransversely within the confined space with the largest transversedimension of each device longitudinally displaced from the largesttransverse dimension of the device or devices disposed adjacent thereto,whereby the high sound frequencies of the said band of frequenciesbecome attenuated during the travel of the medium along the resultingzig-zag path or paths, and a smooth lining extending from the largest tothe smallest transverse dimensions of each device to streamline the flowof the medium along the said zig-zag path or paths.

5. In an acoustic system in which a fluid medium is adapted to flowlongitudinally through a confined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the fiowof the medium comprising a plurality of longitudinally extendingsound-absorbing devices each of varying transverse dimension spacedtransversely within the confined space with the largest transversedimension of each device longitudinally displaced from the largesttransverse dimension of the device or device disposed adjacent thereto,whereby the high sound frequencies of the said band of frequenciesbecome attenuated during the travel of the medium along the resultingzig-zag path or paths, and a smooth lining disposed between the largestand smallest transverse dimensions of each device to stream-line theiiow of the medium along the said zig-zag path or paths, the largesttransverse dimensions of each device being separated from one another adistance corresponding substantially to the halfwavelength ofintermediate sound frequencies of the said band offrequencies in orderto attenuate the said intermediate frequencies.

6. In an acoustic system in which air is adapted to iiow longitudinallythrough a confined space, means for reducing the intensity of soundenergy of a band of frequencies accompanying the ow of the aircomprisinga plurality of longitudinally extending sound-absorbing devices each ofperiodically varying transverse dimension substantially equally spacedtransversely within the confined space with the largest transversedimension of each device disposed substantially opposite to the smallestdi-v mension ofthe devices disposed adjacent thereto, whereby the highsound frequencies of the said band of frequencies become attenuatedduring the travel of the air along the resulting zig-zag paths, thelargest transverse dimensions of each device being separated from oneanother a distance corresponding substantially to the half wavelength ofintermediate sound frequencies of the said band of frequencies in orderto attenuate the said intermediate frequencies.

7. In an acoustic system in which air is adapted to flow longitudinallythrough a confined space, means for reducing the intensity of soundenergy of a band of frequencies accompanying the fiow of the aircomprising a plurality of longitudinally extending sound-absorbingdevices each of periodically varying transverse dimension substantiallyequal-ly spaced transversely within the confined space with the largesttransverse dimension of each device disposed substantially opposite tothe smallest dimension of the devices disposed adjacent thereto, wherebythe high sound frequencies of the said band of frequencies becomeattenuated during the travel of the air along the resulting zig-zagpaths, and a smooth lining disposed between the largest and smallesttransverse dimensions of each device to stream-line the flow of the airalong the said zig-zag paths, the largest transverse dimensions of eachdevice being separated from one another a distance correspondingsubstantially to the half-wavelength of intermediate sound frequenciesof the said band of frequencies in order to attenuate the saidintermediate frequencies.

8. In an acoustic system in which air is adapted to flow longitudinallythrough a conned space, means for reducing the intensity of sound energyof a band of frequencies accompanying the iiow of the air comprising aplurality of longitudinally extending soundaabsorbing devices each ofperiodically varying transverse dimension substantially equally spacedtransversely within the confined space with the largest transversedimension of each device disposed substantially opposite to the smallestdimension of the devices disposed adjacent thereto, Whereby the highsound frequencies of the said band of frequencies become attenuatedduring the travel of the air along the resulting zig-Zag paths, and asmooth acoustically transparent lining disposed between the largest andsmallest transverse dimensions of each device to stream-line the ow ofthe air along the said zig-zag paths, the largest transverse dimensionsof each device being separated from one another a distance correspondingsubstantially to the half-wavelength of intermediate sound frequenciesof the said band of frequencies in order to attenuate the saidintermediate frequencies.

9. In an acoustic system in which air is adapted to iiow longitudinallythrough a confined space, means for reducing the intensity -of soundenergy of a band of frequencies accompanying the ow of the aircomprising a plurality of longitudinally extending sound-absorbingdevices each of substantially uniform transverse dimension and providedwith a plurality of separated sound-absorbing baies of greatertransverse dimension, the devices being substantially equally spacedtransversely within the confined space with the baffles of each devicedisposed between the baiiies of the devices disposed adjacent thereto,whereby the high sound frequencies of the said band of frequenciesbecome attenuated during the travel of the air along the resultingzig-Zag paths, the separation of the baflies of each device from oneanother corresponding substantially to the half-wavelength ofintermediate sound frequencies of the said band of frequencies in orderto attenuate the said intermediate frequencies.

l0. In an acoustic system in which air is adapted to flow longitudinallythrough a confined space, means for reducing the intensity of soundenergy of a band of frequencies accompanying the flow of the aircomprising a plurality of longitudinally extending sound-absorbingdevices each of substantially uniform transverse dimension and providedwith a plurality of separated sound-absorbing baffles of greatertransverse dimension, the devices being substantially equally spacedtransversely within the confined space with the bafes of each devicedisposed between the bafiies of the devices disposed adjacent thereto,whereby the high sound frequencies of the said band of frequenciesbecome attenuated during the travel of the air along the resultingzig-zag paths, and an acoustically transparent lining disposed betweenthe baffies of each device along the said zig-zag paths to stream-linethe fiow of air therealong, the separation of the bafiies of each devicefrom one another corresponding substantially to the half-Wavelength ofintermediate sound frequencies of the said band of frequencies in orderto attenuate the said intermediate frequencies.

ll. In an aerodynamic test chamber in which air is adapted to flowlongitudinally through the chamber in response to the rotation of apropeller and the like within the chamber, means disposed at each end ofthe test chamber for reducing the intensity of sound energy of a band offrequencies generated by the rotation of the propeller and the like andaccompanying the flow of the air comprising a plurality oflongitudinally extending sound-absorbing devices each of periodicallyvarying transverse dimension spaced transversely within the ends of thetest chamber with the largest transverse dimension of each devicedisposed substantially opposite to the smallest dimension of the deviceor devices disposed adjacent thereto, whereby the high sound frequenciesof the said band of frequencies become attenuated during the travel ofthe air along the resulting zig-zag paths, and a smooth lining disposedbetween the largest and smallest transverse dimensions of each device tostream-line thev iiow of the air along the said zig-zag path or paths,the largest transverse dimensions of each device being separated fromone another distance corresponding substantially to the half-wavelengthof intermediate sound frequencies of the said band of frequencies inorder to atten uate the said intermediate frequencies.

12. In an aerodynamic test chamber in which air is adapted to flowlongitudinally through the chamber in response to the rotation of apropeller and the like within the chamber, means disposed at each end ofthe test chamber for reducing the intensity of sound energy of a band offrequencies generated by the rotation of the propeller and the like andaccompanying the tiow of the air comprising a plurality oflongitudinally extending soundabsorbing devices each of substantiallyuniform transverse dimension and provided with a plurality of separatedsound-absorbing baffles of greater transverse dimension, the devicesbeing substantially equally spaced transversely within the ends of thetest chamber with the baies of each device disposed between the bafflesof the devices disposed adjacent thereto, whereby the high soundfrequencies of the said band of frequencies become attenuated during thetravel of the air along the resulting zig-zag paths, and an acousticallytransparent lining disposed between the bafiies of each device along thesaid zig-zag paths to stream-line the flow of air` therealong, theseparation of the baffles of each device from one another correspondingsubstantially to the half-wavelength of intermediate sound frequenciesof the said band of frequencies in order to attenuate the saidintermediate frequencies.

13. An apparatus as claimed in claim l2 and in which the transversespacing between the said longitudinally extending sound-absorbingdevices is from about 2 to about 6 feet, and the separation of thebafiies of each device from one another is from about 4 to l2 feet.

14. A method of the character described that comprises causing soundenergy of a band of frequencies to travel in a fluid medium along azig-zag path of varying transverse dimensions, absorbing the high soundfrequencies of the said band of frequencies as the sound energy travelsalong the said zig-zag path, and fixing the suc- 8 cessive crests of thepath substantially in accordance with the half-wavelength of theintermediate sound frequencies of the said band of frequencies in orderto resonate the said intermediate frequencies, thereby to attenuate thesaid intermediate frequencies.

15. A method of the character described that cornprises causing soundenergy of a band of frequencies to travel in air along a zig-zag path ofvarying transverse dimensions, absorbing the high sound frequencies ofthe said band of frequencies as the sound energy travels along the saidzig-zag path, and fixing the successive crests of the zig-zag pathsubstantially in accordance with the half- Wavelength of theintermediate sound frequencies of the said band of frequencies in orderto resonate the said intermediate frequencies, thereby to attenuate thesaid intermediate frequencies.

16. A method of the character described that comprises causing soundenergy of a band of frequencies to travel in air along a zig-zag path ofvarying transverse dimension, stream-lining the air flow along theZig-zag path, absorbing the high sound frequencies of the said band offrequencies as the sound energy travels along the said zig-zag path, andfixing the successive crests of the Zigzag path substantially inaccordance with the half-wavelength of the intermediate soundfrequencies of the said band of frequencies in order to resonate thesaid intermediate frequencies, thereby to attenuate the saidintermediate frequencies.

17. A method of the character described that comprises causing soundenergy of a band of frequencies to travel in air along a plurality ofsuccessively converging and diverging zig-Zag paths of varyingtransverse dimensions, absorbing the high sound frequencies of the saidband of frequencies as the sound energy travels along the said zig-zagpaths, and fixing the successive crests of each zigzag pathsubstantially in accordance with the half-wavelength of the intermediatesound frequencies of the said band of frequencies in order to resonatethe said intermediate frequencies, thereby to attenuate the saidintermediate frequencies.

18. In an acoustic system in which a fluid medium is adapted to fiowlongitudinally through a confined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the flowof the rnedium comprising a plurality of longitudinally extendingsound-absorbing devices of varying transverse dimensions spacedtransversely within the confined space to provide a path or paths ofsuccessively increasing and decreasing transverse dimensions throughwhich high sound frequencies of the said band of frequencies are forcedto travel in zig-zag fashion, whereby the high sound frequencies of thesaid band of frequencies become attenuated during such travel,successive regions of decreased transverse dimensions of each path orpaths being separated from one another by a spacing resonant to theintermediate soundfrequencies of the said band of frequencies in orderattenuate the said intermediate frequencies.

19. In an acoustic system in which a fluid medium is adapted to flowlongitudinally through a confined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the flowof the medium comprising'a plurality of longitudinally extendingsoundabsorbing devices spaced transversely within the confined space andshaped to force the high sound frequencies of the said band offrequencies to travel in a zig-zag path or paths therebetween, wherebythe high sound frequencies of the said band of frequencies becomeattenuated during such travel, and means for providing a high impedanceto the flow of the sound energy along the zigzag path or paths atintervals spaced longitudinally therealong a distance correspondingsubstantially to the halfwavelength of intermediate sound frequencies ofthe said band of frequencies in order to attenuate the said intermediatefrequencies.

20. In anacoustic system in which a iiuid medium is adapted to llowlongitudinally through a confined space, means for reducing theintensity of sound energy of a band of frequencies accompanying the flowof the medium comprising a plurality of longitudinally extendingsoundabsorbing devices spaced transversely within the confined space andof periodically varying transverse-dimensional shape to force the highsound frequencies of the said band of frequencies to travel in a zig-zagpath or paths therebetween, whereby the high sound frequencies of thesaid band of frequencies become attenuated during such travel, thesuccessive direction-changing points of the zig-zag path or paths beingspaced longitudinally a distance corresponding substantially to thehalf-wavelength of intermediate sound frequencies of the said band offrequencies in order to provide a high impedance to the flow of thesound energy at intervals spaced longitudinally a distance correspondingto the said half-wavelength, thereby to attenuate the said intermediatefrequencies.

21. In an acoustic system in which air is adapted to flow longitudinallythrough a confined space, means for reducing the intensity of soundenergy of a band of frequencies accompanying the ow of the aircomprising a plurality of longitudinally extending sound-absorbingdevices each provided with a plurality of separated soundabsorbingbalfles of greater transverse dimension than other portions of thedevices, the devices being substantially equally spaced transverselywithin the confined space with the baffles of each device disposedbetween the ballles of the devices disposed adjacent thereto, wherebythe high sound frequencies of the said band of frequencies becomeattenuated during the travel of the air along the resulting zig-zagpaths, the separation of the bacs of each device from one anothercorresponding substantially to the half-wavelength of intermediate soundfrequencies of the said band of frequencies in order to attenuate thesaid intermediate frequencies.

22. In an acoustic system in which air is adapted to ilow longitudinallythrough a conned space, means for reducing the intensity of sound energyof a band of frequencies accompanying the ow of the air comprising aplurality o-f longitudinally extending sound-absorbing devices eachprovided with a plurality of separated soundabsorbing baffles of greatertransverse dimension than other portions of the devices, the devicesbeing substantially equally spaced transversely within the confinedspace with the battles of each device disposed between the bales of thedevices disposed adjacent thereto, whereby the high sound frequencies ofthe said band of frequencies become attenuated during the travel of theair along the resulting zig-zag paths, and an acoustically transparentlining disposed between the bafes of each device along the said zig-zagpaths to stream-line the ow of air therealong, the separation of theballes of each device from one another corresponding substantially tothe half-wavelength of intermediate sound frequencies of the said bandof frequencies in order ot attenuate the said intermediate frequencies.

23. In an aerodynamic test chamber in which air is adapted to ilowlongitudinally through the chamber in response to the rotation of apropeller and the like within the chamber, means disposed at each end ofthe test chamber for reducing the intensity of sound energy of a band offrequencies generated by the rotation of the propeller and the like andaccompanying the ow of the air comprising a plurality of longitudinallyextending sound-absorbing devices each provided with a plurality ofseparated sound-absorbing baflles of greater transverse dimension thanother portions of the devices, the devices being substantially equallyspaced transversely within the ends of the test chamber with the bafllesof each device disposed between the baffles of the devices disposedadjacent thereto, whereby the high sound frequencies of the said band offrequencies become attenuated during the travel of the air along theresulting zig-zag paths, and an acoustically transparent lining disposedbetween the baffles of each device along the said zig-zag paths tostream-line the flow of air therealong, the separation of the baffles ofeach device from one another corresponding substantially to thehalf-wavelength of intermediate sound frequencies of the said band offrequencies in order to attenuate the said intermediate frequencies.

24. An apparatus as claimed in claim 23 and in which the transversespacing between the said longitudinally extending sound-absorbingdevices is from about 2 to about 6 feet, and the separation of thebaflles of each device from one another is from labout 4 to 12 feet.

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